Phase equilibrium condition measurements in carbon dioxide hydrate forming system coexisting with seawater
Graphical abstract
Introduction
Water shortage caused by the population growth together with the expansion of industrial and agricultural activities is one of the most important threats for human society and sustainable development [1], [2]. To overcome this threat, dam building, water well construction, wastewater reuse, and seawater desalination have been utilized [1], [3]. Seawater desalination is an attractive technology to increase available fresh water for drinking, industrial consumption, and agricultural usage [4], [5], [6] because 97.5% of the earth's water is seawater. Many seawater desalination technologies such as multi-stage flash distillation [7], [8], multiple-effect distillation [9], [10], and reverse osmosis [11], [12] processes have been researched and developed. Although these technologies are mature, the research and development have been continued to search more efficient process for seawater desalination [4], [13], [14]. The desalination technology by clathrate hydrate formation and decomposition is one of the possible novel desalination processes [15], [16].
Clathrate hydrates (abbreviated hydrates hereafter) are guest–host solid compounds composed of host water molecules and guest molecules [17]. The host water molecules form cage-like structure while the guest molecules occupy it. Light hydrocarbons and noble gases are typical guest compounds. Carbon dioxide is also a typical guest compound that form hydrates under relatively low pressure and high temperature conditions [18]. It is well known that electrolytes in seawater do not enter the hydrate structure. Thus, the desalination technology could be performed based on the hydrate formation and decomposition [15], [16]. If hydrate crystals formed from seawater and a guest compound that is immiscible with seawater, there exist three phases: liquid phase having high electrolytes concentration, electrolytes-free solid hydrate phase, and guest-rich liquid or gas phase. After the separation of the hydrate phase from the other two fluid phases, pure water could be obtained by the decomposition of the hydrate crystals followed by the separation of the guest compound. The separated guest compound could be reused for the hydrate formation. The guest compound should be hydrophobic and form hydrate at moderate pressure and temperature conditions. The candidates of guest compounds for seawater desalination technology is hence cyclopentane [16], [19], propane [20], hydrofluorocarbons [21], [22], and carbon dioxide [23], [24], [25]. Among the guest compounds, carbon dioxide has advantages such as low cost for production, safe for potable water [23], [24], [25]. Although the hydrate-based seawater desalination process was first proposed in 1940 s [26], it has not been commercialized due to the several challenges such as the speed of hydrate formation, separation of the hydrate crystals from the remaining seawater, and the recovery of the guest compounds from the produced pure water [15], [27]. However, the hydrate-based seawater desalination process was revisited in 2010s [15], [16], [19], [20], [21], [22], [23], [24] owing to the recent progress of the understanding of the hydrate-related phenomena and the expanding water demand.
In the carbon dioxide hydrate forming system, (liquid + hydrate + gas) three-phase equilibrium pressure–temperature conditions determine the stability zone for hydrate crystals. Here, liquid phase corresponds to the aqueous solution of electrolytes containing carbon dioxide. The concentrations of the electrolytes are the valuables while the carbon dioxide is dissolved up to the saturation concentration. The phase equilibrium conditions depend on the concentrations of electrolytes. In the seawater desalination process, the concentrations of electrolytes increase with the formation of hydrate crystals. Thus, the phase equilibrium conditions correspond to concentrated seawater is essential for the process design. The three-phase equilibrium conditions in the carbon dioxide hydrate forming system coexisting with various concentrations of aqueous solution of single or multiple electrolytes have been widely measured and used to make correlations [25], [28], [29], [30], [31], [32], [33], [34]. However, the three-phase equilibrium conditions coexisting with seawater have been scarcely reported [28], [35], [36] and the reported concentrations of electrolytes are limited although the operation condition of seawater desalination plant is determined based on the phase equilibrium conditions in seawater system. In the present study, the three-phase equilibrium conditions in the carbon dioxide hydrate forming system coexisting with synthetic standard seawater were measured. The three concentrations of electrolytes in the synthetic standard seawater were applied in the experiments. One corresponds to the normal concentration of electrolytes in seawater and the others mimic the concentrated seawater.
Section snippets
Materials
The specifications of the materials used in the experiments are listed in Table 1. The sample fluids were research-grade carbon dioxide gas supplied from Japan Fine Products Co. Ltd. and pure water prepared in our laboratory. The carbon dioxide gas has certified purity of 0.99995 in mole fraction. The pure water was produced by treating tap water with a reverse osmosis water purifier (model RFP742HA, Toyo Roshi Kaisha, Ltd.) that is comprised of an ion-exchange devise and a reverse osmosis
Results and discussion
The determined (Lss + H + Vg) three-phase equilibrium pressure–temperature conditions are listed in Table 3. Here, Lss is the synthetic standard seawater-rich liquid phase. Carbon dioxide is dissolved up to the saturated solubility in Lss. H is the solid carbon dioxide hydrate phase composed of water and carbon dioxide while it excludes the electrolytes. Vg is the carbon dioxide-rich gas phase. The vapour pressure of water and electrolytes exist in Vg although the vapour pressure of each
Conclusions
The experimental determination of the equilibrium conditions was performed in the carbon dioxide hydrate forming system coexisting with synthetic standard seawater to describe the required thermodynamic conditions for the hydrate-based seawater desalination technology. The synthetic standard seawater was synthesized using seven salts and water. The total mass fraction of electrolytes in the synthetic seawater was wss = 0.0351, 0.0700, or 0.1036. The equilibrium temperature and pressure ranges
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by JSPS KAKENHI Grant Number JP17K14604.
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